My guess would be that the solar charger has little or no regulation and could overcharge the battery. Most devices have charging regulators and may turn off once the battery is charged. Since I don't know what make of solar charger we're talking about, I can only guess at the reason, but overcharging will shorten the life of a battery. I gather that the solar charger has an internal battery that is used to charge other devices and that the solar panel charges the internal battery. This is convenient, because you don't have to leave the device that you want to charge hooked to the solar charger and laying in the sun. I have one like this that I bought several years ago and have regretted buying ever since. The internal battery is large and heavy, while the solar charger is small and underpowered. I seldom get more than half a charge on the solar cell and need to plug the battery in to a USB charger to get a full charge. The internal battery has to put out 5 Volts even if the device being charged only has a 3 Volt battery because the USB voltage is 5 Volts. The rest of the energy is wasted as heat. I'm really not up on the state of the art solar chargers, but this is an area that will gain importance in a connected world.